The invention relates to a semi-spherical shoe, and in particular to a semi-spherical shoe which is preferred to be interposed between a piston and a swash plate of a swash plate compressor, for example.
The use of a semi-spherical shoe having a semi-spherical surface which is fitted into a semi-spherical recess formed in a piston and an end face which is disposed in sliding contact with a flat surface of a swash plate in a swash plate compressor is known in the art.
Several arrangements are known in the art which are directed to preventing a seizure when a swash plate compressor is started or at low temperatures, by reducing a sliding resistance between the semi-spherical shoe and the piston. Specifically, in a first arrangement, a top portion of the semi-spherical surface is formed with a curvature which is greater than the curvature of the semi-spherical recess to form a space between the top portion and the semi-spherical recess which acts as a reservoir for a lubricant oil. In a second arrangement, the semi-spherical surface of the semi-spherical shoe is cut through a top portion thereof to provide a flat surface so that a space may be provided between the flat surface and the semi-spherical recess in the piston to provide a reservoir of lubricant oil. In a third arrangement, an axially depressed opening is formed in the top portion of the semi-spherical surface to form a space between the opening and the semi-spherical recess in the piston to serve as a reservoir for a lubricant oil.
However, in the first and the second arrangement, the space which is intended to be a reservoir of lubricant oil has a reduced volume, allowing only a limited quantity of oil to be maintained in the space.
In the third arrangement, the space formed between the opening and the semi-spherical recess has an increased volume to allow an increased quantity of oil to be maintained therein, but a flow of the lubricant oil from within the space to a region of sliding contact which is disposed externally of the space is degraded. As a consequence, there result disadvantages that it is difficult to change the lubricant oil in the space and that an accumulation of abraded powder is likely to occur in the opening.
In view of the forgoing, it is an object of an invention to provide a spherical shoe in which the volume of the space mentioned above is increased in comparison to the first and second arrangement mentioned above while improving the flow of the lubricant oil between inside and outside the space in comparison to the third arrangement.
At this end, in a semi-spherical shoe having a semi-spherical surface which is fitted into a semi-spherical recess of a first movable member and an end face which is disposed in sliding contact with a flat surface of a second movable member, in accordance with the invention, a spherical recess is formed in a top portion of the semi-spherical surface.
With this arrangement, a space may be formed between the spherical recess and the semi-spherical recess which has an increased volume, and the smoothness of the spherical and the semi-spherical recess which form together the space allows the lubricant oil in the space to be smoothly changed. In this manner, an accumulation of abraded powder in the spherical recess can be prevented in a favorable manner while increasing the volume of the space which serves as a reservoir of lubricant oil.